The natural tendency of a soil to lose moisture and nutritive properties calls for preventive measures i.e. irrigation and fertilization. In this context, it is important to provide fertilizers that have little or no cytotoxicity nor phytotoxicity, and/or substances that can enhance the water retention capacity of soil for a relatively long period of time, in the order of, say, up to two weeks, between water supplies.
It would be advantageous to find substances that can be used as "total" soil conditioners i.e. would both enhance the water retention and provide important nutrients to soil without excessive plant toxicity.
The use of synthetic and natural zeolites as soil conditioners has been studied extensively over the last few years. It has been found that certain zeolites meet two of the above criteria, i.e. they are effective in enhancing the moisture retention of soil and in releasing nutritive ions such as potassium and ammonium to the soil, the ions being incorporated in the zeolite structure.
Such materials have been described in detail in U.S. Pat. No. 4,810,280 to Le Van Mao et al. The patent proposes to use, by admixing with soil, chryso-zeolites produced by acid leaching of chrysotile asbestos and by subsequent impregnation of the resulting solids with an alkali metal hydroxide, notably sodium hydroxide, in an aqueous solution. The production of chryso-zeolites, compounds which retain both the asbestos and zeolite structures, is described in U.S. Pat. No. 4,511,667 to Le Van Mao, et al and in a publication entitled "Leached asbestos materials: precursors of zeolites", Zeolites 1989, vol. 9, 405-411. These references teach the acid leaching step followed by the alkali metal impregnation.
While the method and materials of U.S. Pat. No. 4,810,280 are useful, the zeolites produced and used according to that patent still contain considerable amounts of sodium which is practically almost impossible to substitute to a large extent by other cations e.g. potassium by way of ion exchange. This is probably due to the fact that above certain content limit, sodium in the impregnation step (see above) becomes incorporated both in the zeolite crystalline lattice and in the asbestos matrix. The latter position is much more resistant than the former one, to substitutions by ion exchange without disrupting the zeolite structure.
Phytotoxicity of sodium ions is well known and is a negative factor in the widespread agricultural use of the chryso-zeolites of the U.S. Pat. No. 4,810,280.
Accordingly, there is still a need for materials useful in agriculture for rapid and prolonged storage of water in soils, the materials presenting little or no plant toxicity problems and effective in releasing valuable nutrients to plant roots.